Mitt Romney won a clear victory in the Illinois primary on Tuesday, bolstering
his claim to be the only viable Republican candidate to take on Barack Obama
in November.

Returns from 98 per cent of Illinois' precincts in Tuesday's primary showed Mr Romney, the former Massachusetts governor, gaining 47 per cent of the vote compared to 35 per cent for Rick Santorum, 9 per cent for Texas Rep. Ron Paul and 8 per cent for a fading former House Speaker Newt Gingrich.

After failing to defeat his ultra-conservative Republican opponent Mr Santorum in the southern states of Alabama and Mississippi a week ago, Mr Romney’s win in Illinois provides a much-needed fillip to a lacklustre campaign.

In his victory speech, the former Massachusetts governor barely mentioned his remaining Republican rivals focusing instead on Mr Obama, who he charged with strangling the American spirit of innovation.

"Day by day, job-killing regulation by job-killing regulation, bureaucrat by bureaucrat, this president is crushing the dream and the dreamers," Mr Romney said.

He contrasted his own career in private business with Mr Obama's pre-politics stint as a constitutional law professor and warned that "economic freedom" itself was at stake in November's election.

But even in victory Mr Romney seemed to confirm some of the fears about his viability as a general election candidate. He stumbled repeatedly in his speech, at one point saying he looked ahead to an America that was "humbled", before correcting himself to say "humble".

Even Mr Romney’s own supporters, including the 2008 nominee John McCain, have said publicly that their relatively moderate candidate needs to “do better” and lacks the spark needed to enthuse voters.

Despite his defeat, Mr Santorum promised to stay in the race in the belief that he can play the spoiler and stop Mr Romney from reaching the “magic number” of 1,144 that would win him the nomination outright. Mr Romney now has 563 delegates. Mr Santorum has 263, Mr Gingrich 135 and Mr Paul 50.

Speaking near Gettysburg, Pennsylvania - the site of the bloodiest battle of the American Civil War - Mr Santorum said the US faced its most important election since 1860, when it elected Abraham Lincoln on the eve of civil war.

"The foundational issue in this race, one that is in fact the cause of the other maladies that we're feeling - whether it's in the economy or the budget crisis - boils down to one word: freedom," he said.

Newt Gingrich looked set to come a humiliating last place, falling behind Ron Paul, the libertarian congressman who has openly said he has little chance of winning the nomination.

The loss leaves Mr Santorum fighting once again to maintain the credibility of a campaign which has been drained of momentum in recent days by a series of gaffes and organisational failures.

A day before yesterday’s vote Mr Santorum, who relies on support from the Republican party’s evangelical base, had talked of a scoring a “huge or surprise win” that would have “guaranteed” him the nomination.

But in a damaging 48 hours ahead of the Illinois primary, Mr Santorum found himself on the defensive after letting slip that he “didn’t care” about the unemployment rate.

The clumsy remarks - which were intended to appeal to conservatives concerned about social issues and religious freedom - were seized upon by Mr Romney.

“One of the people who is also running for the Republican nomination said that he doesn't care about the unemployment rate; that does bother me,” Mr Romney said gleefully, “I do care about the unemployment rate. It does bother me. I want to get people back to work.”

Later, Mr Santorum said of remarks that he wished he “had a do-over”, but political analysts say that such amateurishness and indiscipline, which has always plagued the shoestring Santorum campaign, is starting to tell.

Strategists also say Mr Santorum made a serious mistake wasting two days of the Illinois race by campaigning in Puerto Rico last week.

He was also unable to win delegates in four of Illinois’s 19 congressional districts after his campaign failed to register before electoral deadlines expired.

Mr Santorum’s fiery brand of Catholicism has proved a double-edged sword, stirring support among rural voters and religious evangelicals but deterring more moderate urban voters, including fellow Catholics who have repeatedly backed Mr Romney.

Mr Santorum was forced to defend his ultra-conservative views after video footage surfaced showing him applauding a Baptist pastor at a church service in Louisiana last Sunday after the minister delivered a rant against gay people, liberals and women who have abortions.